Method for control hand-off of CDMA system, base station used therefor, and base station controller

ABSTRACT

A handoff method of a CDMA mobile communication system in which one or a plurality of tiers of wireless frequencies are assigned on an individual wireless zone basis, locational information of a mobile device is obtained, and when, according to the foregoing locational information, the mobile device is present in the vicinity of a frequency-tier boundary where different frequency tiers are assigned to a cell conducting a communication and an adjacent cell, an inter-frequency handoff is performed to a tier of a common frequency prepared commonly in both the foregoing cell conducting the communication and the foregoing adjacent cell.

TECHNICAL FIELD

[0001] The present invention relates to a handoff control method of aCDMA system, and a base station and a base station control apparatusused therefor, and more particularly, to a handoff control method of aCDMA system, and a base station and a base station control apparatusused therefor in a case where a cellular mobile communication systemusing a CDMA (Code Division Multiple Access) method has a plurality offrequency tiers.

BACKGROUND ART

[0002] In a CDMA cellular mobile communication system, a plurality offrequency tiers according to traffic intensity are prepared. Here, onefrequency band subjected to a frequency diffusion is referred to as onefrequency tier. For example, an operation conformation is adopted whichprepares a large number of frequency tiers in an area, such as an urbanarea, where traffic is relatively high, and prepares a small number offrequency tiers in an area, such as a suburban area, where traffic isrelatively low.

[0003] In a CDMA digital cellular mobile communication system, in a caseof having a plurality of frequency tiers, a method has beenconventionally used at a frequency-tier boundary representing a boundaryfor each frequency; in the method, a device transmitting to a mobiledevice only control signals, such as pilot signals corresponding to thenumber of frequency tiers identical to the tiers in each of the cells isinstalled in a base station in an adjacent cell so as to cause themobile device to report of a destination cell and to switch (to handoff) to a frequency prepared in the adjacent cell.

[0004] Alternatively, another method has been used in which a basestation in an adjacent cell is provided with a device capable ofmeasuring an electric field intensity from a mobile device using afrequency not assigned to the cell so that the system recognizes adestination cell from the measurement result, and causes a handoff to afrequency that the adjacent cell prepares.

[0005] However, in a case where the present sell has only a tier of afrequency A and does not prepare a tier of a frequency B whereas theadjacent cell has the tiers of the frequencies A and B, a device (abeacon device) transmitting only a control signal with respect to amobile device, such as a pilot signal of the tier of the frequency B,becomes necessary. Alternatively, a device measuring an electric fieldintensity from a mobile device using the tier of the frequency B notexisting in the present cell needs to be prepared. Accordingly, thesecases have had problems, such as that expenses for correspondentinstallations become necessary.

[0006] Secondly, there is a problem that a time before the execution ofthe handoff becomes longer, and as a result of this, a time during whichan audio quality deteriorates becomes longer. This is because, in a caseof measuring an electric field intensity with respect to a transmittedsignal from a mobile device, the base station performs an operation ofmeasuring the electric field intensity of the signal receivable from themobile device after receiving from the mobile device a deteriorationreport to the effect that a sound has deteriorated.

[0007] Additionally, in a case of measuring the electric field intensityof the signal receivable from the mobile-device in all neighboring cellsso as to determine a destination cell, the time before the execution ofthe handoff becomes even longer. Therefore, there has been a problem ofan increasing likelihood that the mobile device moves out of a servicearea before completion of the handoff so that the communication isdisconnected.

[0008] In addition, as described in Japanese Laid-Open PatentApplication No. 6-326653, there is a method that uses a tier of a commonfrequency A only in switching channels in communication and in setting aline upon sending and receiving a call, and uses tiers of frequencies Band C as calling channels. In this case, a usability of the tier of thecommon frequency A is aggravated, and additionally, installationexpenses of the base station increase. Further, there has been a problemthat, because of an increase in the number of times the frequencies areswitched, the number of times of hard handoffs involving a momentarydiscontinuation increases, so as not to take full advantage of a softhandoff involving no momentary discontinuation which is a characteristicof the CDMA.

[0009] Further, as described in Japanese Laid-Open Patent ApplicationNo. 10-155173, in a case of switching frequencies-in-use in response toa deterioration report regarding the frequency being used from themobile device, since a trigger of a handoff is a deterioration report ofa pilot signal, a handoff control sometimes does not finish as to amobile device moving at high speed. Also, there has been a problem that,since the frequency switching operation is sometimes performed in a casewhere the mobile device moves from a cell of the frequency-tier boundaryto an inner cell (a cell not of the frequency-tier boundary),opportunities for the hard handoffs involving a momentarydiscontinuation increase.

DISCLOSURE OF INVENTION

[0010] It is a general object of the present invention to provide ahandoff control method of a CDMA system which is free from a risk of acommunication being disconnected upon an inter-cell handoff switchingfrom a cell conducting the communication to a destination cell, cansuppress a capital investment upon a system construction, can shorten aprocessing time unto a completion of a handoff, and can realize anefficient operation of the system as a whole.

[0011] In order to achieve this object, the present invention isarranged such that, in a handoff method of a CDMA mobile communicationsystem in which one or a plurality of tiers of wireless frequencies areassigned on an individual wireless zone basis,

[0012] locational information of a mobile device is obtained, and when,according to the locational information, the mobile device is present inthe vicinity of a frequency-tier boundary where different frequencytiers are assigned to a cell conducting a communication and an adjacentcell, an inter-frequency handoff is performed to a tier of a commonfrequency prepared commonly in both the cell conducting thecommunication and the adjacent cell.

[0013] According to the handoff control method of the CDMA system asabove, there is no risk of a communication being disconnected upon aninter-cell handoff switching from a cell conducting the communication toa destination cell, and there is no need for installing a devicetransmitting only a control signal, such as a pilot signal of afrequency not prepared in each of the cells, or for installing a devicemeasuring an electric field intensity of a mobile device; therefore, acapital investment upon a system construction can be suppressed.

BRIEF DESCRIPTION OF DRAWINGS

[0014] Other objects, features and advantages of the present inventionwill become more apparent from the following detailed description whenread in conjunction with the accompanying drawings.

[0015]FIG. 1 is a system configuration diagram of a first embodiment ofa CDMA digital cellular mobile communication system to which the presentinvention is applied.

[0016]FIG. 2 is a diagram illustrating changes in time of locationalinformation of a mobile device 4.

[0017]FIG. 3 is a flowchart and a sequence of the first embodiment of ahandoff control performed by base stations and a base station controlapparatus.

[0018]FIG. 4 is a flowchart and a sequence of the first embodiment ofthe handoff control performed by the base stations and the base stationcontrol apparatus.

[0019]FIG. 5 is a diagram for explaining an embodiment in whichcartographic information is added to a judgment of a moving direction ofthe mobile device.

[0020]FIG. 6 is a system configuration diagram of a second embodiment ofthe CDMA digital cellular mobile communication system to which thepresent invention is applied.

[0021]FIG. 7 is a flowchart and a sequence of the second embodiment ofthe handoff control performed by the base stations and the base stationcontrol apparatus.

[0022]FIG. 8 is a flowchart and a sequence of the second embodiment ofthe handoff control performed by the base stations and the base stationcontrol apparatus.

[0023]FIG. 9 is a flowchart and a sequence of the second embodiment ofthe handoff control performed by the base stations and the base stationcontrol apparatus.

[0024]FIG. 10 is a flowchart and a sequence of the second embodiment ofthe handoff control performed by the base stations and the base stationcontrol apparatus.

[0025]FIG. 11 is a flowchart and a sequence of the second embodiment ofthe handoff control performed by the base stations and the base stationcontrol apparatus.

[0026]FIG. 12 is a flowchart and a sequence of the second embodiment ofthe handoff control performed by the base stations and the base stationcontrol apparatus.

[0027]FIG. 13 is a system configuration diagram of a third embodiment ofthe CDMA digital cellular mobile communication system to which thepresent 3invention is applied.

[0028]FIG. 14 is a flowchart and a sequence of the third embodiment ofthe handoff control performed by the base stations and the base stationcontrol apparatus.

[0029]FIG. 15 is a flowchart and a sequence of the third embodiment ofthe handoff control performed by the base stations and the base stationcontrol apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

[0030] Hereinbelow, a description will be given of a principle of thepresent invention.

[0031] In the present invention, a location of a mobile device in a cellis specified, and a handoff will be performed based on this locationalinformation. For the purpose of specifying locational information of amobile device, firstly, messages are sent out simultaneously from aplurality of wireless base station apparatuses to the mobile device thelocation of which is desired to be specified. Thereafter, a responsemessage from the mobile device is received by each of the base stations.In CDMA, a time by which a mobile device responds to a message requestfrom a base station is clearly prescribed; therefore, the base stationcan calculate a distance to the mobile device from a time differencebetween the received message and the transmitted message.

[0032] When distances to a particular mobile device are specified bythree base stations, a location of the particular mobile device in aservice area can be specified accurately. A method of specifying thelocation of the mobile device according to the distances from each ofthe base stations to the mobile device is based on a principle similarto a so-called triangulation. Therefore, when distances from four basestations to the mobile device can be specified, an altitude of themobile device can be specified as a matter of course.

[0033] In the present invention, the locational information of themobile device obtained as above is intended to be used as a trigger fora handoff. Firstly, a direction in which the mobile device moves isspecified according to changes in time of the locational information,and when the mobile device is moving in a direction toward outside of afrequency tier, a tier of a common frequency prepared both in a cellconducting a communication and in a destination cell is selected as afrequency tier used beforehand, and an inter-frequency handoff switchingfrequency tiers in the cell conducting the communication is executed byusing the selected tier of the common frequency, and thereafter, aninter-cell handoff switching from the present cell conducting thecommunication to the destination cell is executed.

[0034] Therefore, there is no risk of the communication beingdisconnected upon the inter-cell handoff switching from the cellconducting the communication to the destination cell, and there is noneed for installing a device transmitting only a control signal, such asa pilot signal of a frequency not prepared in each of the cells, or forinstalling a device measuring an electric field intensity of the mobiledevice; therefore, a capital investment upon a system construction canbe suppressed.

[0035] Secondly, when a mobile device present in a frequency-tierboundary cell uses a frequently used frequency tier existing in both thefrequency-tier boundary cell and a frequency-tier out-of-range cell, thefollowing handoff control is performed for the purpose of effectiveutilization of the frequency tiers. A moving velocity of a mobile deviceis specified from an amount of change of the locational information perunit time, and in regard to a mobile device present in the proximity ofthe center of the cell with the moving velocity being low, a handoff isexecuted such that a communication is performed by using a tier of anuncommon frequency assigned only to the cell conducting thecommunication (the frequency-tier boundary cell). Additionally, evenwhen a mobile device is present in the proximity of a boundary betweenthe cells, an inter-frequency handoff is executed such that acommunication is performed by using the tier of the uncommon frequencyassigned only to the cell conducting the communication, if the movingvelocity is zero, i.e., the mobile device stops.

[0036] Thirdly, in regard to a mobile device present in thefrequency-tier boundary cell using the frequency tier assigned only tothe frequency-tier boundary cell and moving at a high moving velocity, ahandoff is executed such that a communication is performed by using thetier of the common frequency existing in both the frequency-tierboundary cell and the frequency-tier out-of-range cell.

[0037] Thus, conjecturing the moving direction and the moving velocityof the mobile device can shorten a processing time of a handoff, andperforming the brisk handoff control enables a realization of effectiveutilization of each of the frequency tiers, and a realization of anefficient operation of the system as a whole.

[0038] Hereinbelow, a description will be given, with reference to thedrawings, of embodiments according to the present invention.

[0039]FIG. 1 is a system configuration diagram of a first embodiment ofa CDMA digital cellular mobile communication system to which the presentinvention is applied. In this figure, since necessary traffic intensityis high in base stations 1 and 2, two tiers of two frequencies A and Bare prepared, and since necessary traffic intensity is low in a basestation 3, one tier of one frequency A is prepared. These base stations1 to 3 are connected to a base station control apparatus 5, and the basestation control apparatus 5 stores frequency composition information ofeach of the base stations on an individual base-station basis. A mobiledevice 4 located in a cell that the base station 1 forms moves from thebase station 1 to a cell of the base station 3 while communicating atthe tier of the frequency B.

[0040] Here, a frequency-tier boundary refers to a service area limit ofa cell (the cell of the base station 1) conducting a communication atthe tier of the frequency B, i.e., boundaries K13 and K23 between eachof the cells of the base stations 1 and 2 and the base station 3, in acase where the tier of the frequency B used in the cell conducting thecommunication is not prepared in an adjacent cell (the cell of the basestation 3). From a viewpoint of the mobile device 4 using the tier ofthe frequency B in the cell of the base station 1, the cells of the basestations 1 and 2 are frequency-tier boundary cells, and the cell of thebase station 3 is a frequency-tier out-of-range cell.

[0041]FIG. 3 and FIG. 4 show a flowchart and a sequence of the firstembodiment of a handoff control performed by the base stations and thebase station control apparatus. In step S10 of FIG. 3, for the purposeof specifying a location of the mobile device 4, the base stationcontrol apparatus 5 sends out a message, for example, a Mobile LocationMeasurement Message (hereinafter referred to as MLM message), to themobile device 4, the message being used for specifying the location withrespect to the base stations 1, 2 and 3. The mobile device 4 havingreceived this MLM message sends out a response message after a timestandardized beforehand with respect to each of the base stations, forexample, after a ½ symbol since the reception of the above-mentionedmessage.

[0042] In step S12, each of the base stations 1, 2 and 3 calculates adistance from the base station to the mobile device 4 according to adifference between a time of response with respect to the MLM messagefrom the mobile device 4 and a time of sending out the MLM message tothe mobile device, and reports the distance to the base station controlapparatus 5. Based on the reported result from each of the base stations1, 2 and 3, the base station control apparatus 5 specifies the locationof the mobile device 4 in the service area, and accumulates thislocational information. Then, in step S16, a direction in which themobile device 4 moves is conjectured from the accumulated locationalinformation.

[0043] Besides, it may be arranged that the MLM message is sent out fromone base station 1 to the mobile device 4, and the response message fromthe mobile device 4 is received by the three base stations. Further,arranging that the response message is received by four base stationsenables an altitude of the mobile device to be specified; therefore,further detailed locational information of the mobile device can beobtained. The obtainment of the above-mentioned locational informationmay be performed regularly by the base station control apparatus 5 orthe base stations 1, 2 and 3, or may be performed regularly by the basestation 1, being triggered by a reception of a deterioration report of apilot signal strength measurement message (a Pilot Measurement Message)from the mobile device 4, during a period therefrom until the presentmobile device 4 leaves the base station's own control. The base stationcontrol apparatus 5 can conjecture the direction in which the mobiledevice 4 moves, according to changes in time of the locationalinformation of the mobile device 4, as shown in FIG. 2.

[0044] Here, when it is judged in step S18 that the direction in whichthe mobile device 4 moves is a move toward a direction outside thefrequency-tier boundary (e.g., the boundary K13), and further, in stepS20, when the mobile device 4 is performing a communication by using thetier of the frequency B assigned only within the frequency-tier boundary(within the cells of the base stations 1 and 2), an inter-frequencyhandoff (a hard handoff) is performed to the tier of the frequency Aassigned both within and outside the boundary, in step S22, while themobile device 4 is present within the frequency-tier boundary.Thereafter, in step S24, it is judged whether or not an estimated courseof the mobile device 4 is correct.

[0045] Then, when the mobile device 4 enters the cell of the basestation 31 and there is a handoff request from the mobile device 4 instep S25, an inter-cell handoff (a soft handoff) is performed from thecell of the base station 1 to the cell of the base station 3 at the tierof the same frequency A, in step S26. According to this, even when themobile device 4 moves out of the frequency-tier boundary, thecommunication can be continued without being interrupted. Thereafter, instep S28, it is judged whether or not the mobile device 4 has moved tothe outside of the frequency-tier boundary, and when having moved to theoutside of the frequency-tier boundary, the above-described sequenceends, and transits to another sequence led by the base station 3.Besides, the soft handoff is a handoff causing no momentarydiscontinuation during a call, and the hard handoff is a handoff causinga momentary discontinuation during a call.

[0046] Additionally, cartographic information, such as geographicalfeatures and roads in the service areas.(the cells) in which the basestations 1, 2 and 3 are situated, may be stored in the base stationcontrol apparatus 5 as means for judging the above-mentioned movingdirection of the mobile device 4 so as to enable a detailed conjectureof the moving direction of the mobile device 4 while referring to theforegoing cartographic information. That is, it can be judged in furtherdetail according to road conditions and geographical conditions whetheror not the moving direction of the mobile device 4 is toward a directionoutside the frequency-tier boundary. For example, it is conjectured froma moving direction and a condition of how a road L1 extends that amobile device 6 shown in FIG. 5 is moving toward the outside of thefrequency-tier boundary at a time t4; therefore, the tier of thefrequency A needs to be selected.

[0047] To the contrary, with respect to a mobile device 7, although amoving direction thereof points to the outside of the frequency-tierboundary at a time t3, it is conjectured, from a road condition of aroad L2 curving in the cell of the base station 1, that the mobiledevice 7 is not moving toward the outside of the frequency-tier boundaryat a time t4; therefore, it can be judged that the tier of the frequencyB can be maintained and that there is no need for the hard handoffswitching the frequencies. As described above, combining thecartographic information with the moving-direction judgment of themobile device enables a conjecture of an accurate moving direction, andan effective utilization of the frequently used tier of the commonfrequency A.

[0048]FIG. 6 is a system configuration diagram of a second embodiment ofthe CDMA digital cellular mobile communication system to which thepresent invention is applied. In this figure, the same parts as in FIG.1 are marked with the same marks. In FIG. 6, the tier of the commonfrequency A is assigned to each of the base stations 1, 2 and 3, and thetier of the frequency B is also assigned to the base stations 1 and 2 inwhich necessary traffic intensity is high.

[0049]FIG. 7 to FIG. 12 show flowcharts and sequences of the secondembodiment of the handoff control performed by the base stations and thebase station control apparatus. Among these, FIG. 7 and FIG. 8 show acase where the mobile device 4 is present at a point a in the vicinityof the frequency-tier boundary shown in FIG. 6, FIG. 9 and FIG. 10 showa case where the mobile device 4 is present at a point b in a centerpart of the cell shown in FIG. 6, and FIG. 11 and FIG. 12 show a casewhere the mobile device 4 is present at a point c shown in FIG. 6;identical parts in each case are marked with the same marks.

[0050] In step S30 of FIG. 7, for the purpose of specifying a locationof the mobile device 4, the base station 1 sends out a message, such asthe MLM message, to the mobile device 4, the message being used forspecifying the location. Additionally, the base station 1 notifies thebase stations 2 and 3 around the perimeter of the base station 1 via thebase station control apparatus 5 with a purport that the base station 1has sent out the MLM message with adding a send-out time (step S32). Themobile device 4 having received the foregoing MLM message sends out aresponse message-after a time standardized beforehand with respect toeach of the base stations, for example, after a ½ symbol since thereception of the foregoing message. The base station 1 calculates adistance from the base station 1 to the mobile device 4 according to adifference between a time at which the base station 1 has received theresponse with respect to the MLM message from the mobile device 4 and atime at which the base station 1 has sent out the MLM message to themobile device 4, and reports the distance to the base station controlapparatus 5. In addition, each of the base stations 2 and 3 calculates adistance from the individual base station to the mobile device 4according to a difference between a time at which the base station 1 hassent out the message and a time at which the individual base station hasreceived the response message sent out from mobile device, and reportsthe distance to the base station control apparatus 5.

[0051] It is noted that, in the CDMA, each base station and a basestation control apparatus are synchronized by a GPS (Global PositioningSystem) device. Based on the reported result from each of the basestations 1, 2 and 3, the base station control apparatus 5 specifies thelocation of the mobile device 4 in the service area, in step S36.Further, in step S38, the base station control apparatus 5 accumulatesand analyzes locational information with respect to the mobile device 4reported regularly from each of the base stations so as to accumulatedata, such as locational information, moving directions, and movingvelocities, with respect to the mobile device 4, on the basis of time.

[0052] Here, when the mobile device 4 is present at the point a shown inFIG. 6, and is moving, for example, in a northeast direction at 4 km/h,and if the mobile device 4 is performing a communication by using thetier of the frequency B, the base station control apparatus 5 directsthe base station 1 to perform the hard handoff to the tier of the commonfrequency A (steps S40 to S50 in FIG. 7 and FIG. 8). Accordingly, whenthe mobile device 4 moves to the proximity of the boundary between thebase station 1 and the base station 3, and there is a handoff requestfrom the mobile device 4, the inter-cell handoff (the soft handoff) isperformed from the cell of the base station 1 to the cell of the basestation 3 at the tier of the same frequency A (steps S52 and S54).Thereafter, in step S56, it is judged whether or not the mobile device 4has moved to the outside of the frequency-tier boundary, and when havingmoved to the outside of the frequency-tier boundary, the above-describedsequence ends, and transits to another sequence led by the base station3.

[0053] Additionally, when the mobile device 4 is present also at thepoint a, and stops at the same point, for example, at 0 km/h for 10minutes, and if the mobile device 4 is performing a communication byusing the tier of the common frequency A, the base station controlapparatus 5 directs the base station 1 to perform the hard handoff tothe tier of the frequency B (steps S58 and S59). This enables aneffective utilization of the frequently used tier of the commonfrequency A.

[0054] Here, when the mobile device 4 starts moving again, and themoving direction is northeast, the base station control apparatus 5directs the base station 1 to perform the hard handoff of the frequencyof the mobile device 4 to the tier of the frequency A (steps S48 andS50). By doing this, the tier of the common frequency A can be usedeffectively, and at the same time, the communication can be secured.

[0055] Further, when the mobile device 4 is present at the point b shownin FIG. 6, and is moving, for example, in a northeast direction at 4km/h, and if the mobile device 4 is performing a communication by usingthe tier of the frequency A, the base station control apparatus 5directs the base station 1 to perform the hard handoff to the tier ofthe frequency B for the purpose of effective utilization of the tier ofthe frequency A (steps S60, S42, S62, S63 and S64 in FIG. 9 and FIG.10). Thereafter, when the mobile device 4 approaches the frequency-tierboundary, for example, the point a, while continuing a call, thesequence proceeds to step S40 of FIG. 7, and the base station controlapparatus 5 directs the base station 1 to perform the hard handoff tothe tier of the common frequency A. By doing this, the tier of each ofthe frequencies can be used effectively, and at the same times thecommunication can be secured; and thereafter, even when the mobiledevice 4 moves over to the cell of the base station 3, performing thesoft handoff can maintain the communication. On the other hand, when themobile device 4 is moving at high speed, and is performing acommunication by using the tier of the frequency B, the base stationcontrol apparatus 5 directs the base station 1 to perform the hardhandoff to the tier of the common frequency A (steps S62, S66 and S68).

[0056] In addition, when the mobile device 4 begins a communication atthe point c shown in FIG. 6, and is moving in a northeast direction at ahigh speed, for example, 50 km/h, it is estimated that the mobile device4 soon moves to the cell of the base station 3 outside thefrequency-tier boundary; therefore, the base station control apparatus 5directs the base station 1 to assign the tier of the common frequency Afrom the beginning (steps S70, S42, and S71 to S73 in FIG. 11 and FIG.12). This prevents the communication from being disconnected due to aninter-cell movement in the course of the hard handoff process for themobile device moving at high speed, and even when the mobile device 4makes the inter-cell movement to the cell of the base station 3,performing the soft handoff can maintain the communication. On the otherhand, when the mobile device 4 is moving in the northeast direction at alow speed, and is performing the communication by using the tier of thefrequency A, the base station control apparatus 5 directs the basestation 1 to perform the hard handoff to the tier of the commonfrequency B (steps S62, S74, S76 and S78).

[0057]FIG. 13 is a system configuration diagram of a third embodiment ofthe CDMA digital cellular mobile communication system to which thepresent invention is applied. In this figure, the same parts as in FIG.1 are marked with the same marks. In FIG. 13, the tier of the commonfrequency A is assigned to each of the base stations 1, 2 and 3, and thetier of the frequency B is also assigned to the base stations 1 and 2 inwhich necessary traffic intensity is high.

[0058] In the CDMA, there is a function of continually controlling asignal strength transmitted and received between a base station and amobile device for the purpose of realizing effective utilization of eachfrequency tier. Thereupon, locational information and a presence or anabsence of a movement of the mobile device 4 can be observed in asimplified manner by utilizing a pilot signal strength measurementmessage used for the foregoing control function.

[0059]FIG. 14 and FIG. 15 show a flowchart and a sequence of the thirdembodiment of the handoff control performed by the base stations and thebase station control apparatus.

[0060] In step S80 of FIG. 14, the base station control apparatus 5causes values (α, β, γ) of the pilot signal strength measurementmessages from the mobile device 4 in the base stations 1 to 3 to beimparted so as to calculate a location of the mobile device 4, and instep S82, the base station control apparatus 5 accumulates locationalinformation and a presence or an absence of a movement with respect tothe mobile device 4 reported regularly from each of the base stations.

[0061] Here, assuming that the mobile device 4 present at the point astays at the same point, the pilot signal strength measurement messagesindicate substantially constant values such that the signal strengths(α, γ) from the base station 1 and the base station 3 becomesubstantially identical values. Accordingly, in this case, if the mobiledevice 4 is performing a communication by using the tier of the commonfrequency A, the base station control apparatus 5 directs the basestation 1 to perform the hard handoff to the tier of the frequency B(steps S84 to S87 in FIG. 14).

[0062] Additionally, when the mobile device is moving from the point atoward a northeast direction, the pilot signal strength measurementmessages exhibit a characteristic that the pilot signal strengthmeasurement message from the base station 1 becomes smaller as time goesby, and the pilot signal strength measurement message from the basestation 3 becomes gradually larger as time goes by. In this case, if themobile device 4 is performing a communication by using the tier of thefrequency B, the base station control apparatus 5 directs the basestation 1 to perform the hard handoff to the tier of the commonfrequency A (steps S84 and S85; S92 and S93 in FIG. 15).

[0063] Additionally, assuming that the mobile device present at thepoint b stays at the same point, the pilot signal strength measurementmessages indicate substantially constant values, and only the pilotsignal strength measurement message from the base station 1 is reportedfrom the mobile device 4 to the base station 1 as a relatively largevalue. Accordingly, in this case, if the mobile device 4 is performing acommunication by using the tier of the frequency A, the base stationcontrol apparatus 5 directs the base station 1 to perform the hardhandoff to the tier of the frequency B (step S84; S88 to S91 in FIG.15). Performing the control process as described above enables arealization of effective utilization of each frequency tier.

[0064] Thus, according to the present invention, there is no risk of acommunication being disconnected upon the inter-cell handoff switchingfrom a cell conducting the communication to a destination cell; and inall of cells of the frequency-tier boundary, even when a frequency tierprepared in an adjacent cell is not prepared in each of the cells, acapital investment upon a system construction can be suppressed, becausethere is no need for installing a device transmitting only a controlsignal, such as a pilot signal of the said frequency, or for installinga device measuring an electric field intensity of a mobile device.Further, conjecturing a moving direction and a moving velocity of amobile device can shorten a processing time unto a completion of ahandoff. Additionally, performing a brisk handoff control enables arealization of effective utilization of frequency tiers, and arealization of an efficient operation of the system as a whole.

[0065] Additionally, in the present invention, although the descriptionis given of the configuration in which the subject of the controlsequence is the base station control apparatus, the present invention isnot limited thereto, and the control sequence can be distributed to thebase station and the base station control apparatus.

[0066] Besides, steps S10 and S12 correspond to location conjecturingmeans described in the claims, step S22 corresponds to first handoffmeans, step S16 corresponds to moving-direction conjecturing means, stepS59 corresponds to second handoff means, steps S64 and S78 correspond tothird handoff means, and step S42 corresponds to moving-velocityconjecturing means.

1. A handoff control method of a CDMA system, in the handoff method ofthe CDMA mobile communication system one or a plurality of tiers ofwireless frequencies are assigned on an individual wireless zone basis,wherein locational information of a mobile device is obtained, and when,according to said locational information, said mobile device is presentin the vicinity of a frequency-tier boundary where different frequencytiers are assigned to a cell conducting a communication and an adjacentcell, an inter-frequency handoff is performed to a tier of a commonfrequency prepared commonly in both said cell conducting thecommunication and said adjacent cell.
 2. The handoff control method ofthe CDMA system as claimed in claim 1, wherein a moving direction ofsaid mobile device is conjectured from the locational information ofsaid mobile device, and when the moving direction of said mobile deviceis outward of the frequency-tier boundary, the inter-frequency handoffis performed to the tier of said common frequency.
 3. The handoffcontrol method of the CDMA system as claimed in claim 2, wherein adetailed moving direction is conjectured by combining the locationalinformation of said mobile device and cartographic information of saidcell conducting the communication.
 4. The handoff control method of theCDMA system as claimed in claim 1, wherein, when there is no change inthe locational information of said mobile device, an inter-frequencyhandoff is performed from the tier of said common frequency to a tier ofan uncommon frequency prepared in said cell conducting the communicationand not prepared in said adjacent cell.
 5. The handoff control method ofthe CDMA system as claimed in claim 2, wherein, when said mobile deviceis distant from said frequency-tier boundary according to saidlocational information, an inter-frequency handoff is performed to atier of an uncommon frequency prepared in said cell conducting thecommunication and not prepared in said adjacent cell.
 6. The handoffcontrol method of the CDMA system as claimed in claim 5, wherein amoving velocity of said mobile device is conjectured from an amount ofchange of said locational information, and when said moving velocity isa high velocity, the inter-frequency handoff is performed to the tier ofsaid common frequency.
 7. The handoff control method of the CDMA systemas claimed in claim 1, wherein said locational information is calculatedbased on a time during which a message used for location specificationis transmitted from a base station of said cell conducting thecommunication, and a response message from said mobile device isreceived by each base station.
 8. The handoff control method of the CDMAsystem as claimed in claim 1, wherein said locational information iscalculated based on a pilot signal strength measurement messagetransmitted from said mobile device and received by each base station.9. The handoff control method of the CDMA system as claimed in claim 8,wherein a moving velocity of said mobile device is conjectured based onan amount of change per unit time of the pilot signal strengthmeasurement message received by said each base station.
 10. A basestation and a base station control apparatus of a CDMA system, in thebase station and the base station control apparatus of the CDMA mobilecommunication system one or a plurality of tiers of wireless frequenciesare assigned on an individual wireless zone basis, the base station andthe base station control apparatus of the CDMA system comprising:location conjecturing means for obtaining locational information of amobile device; and first handoff means for performing an inter-frequencyhandoff to a tier of a common frequency prepared commonly in both a cellconducting a communication and an adjacent cell, when, according to saidlocational information, said mobile device is present in the vicinity ofa frequency-tier boundary where different frequency tiers are assignedto said cell conducting the communication and said adjacent cell. 11.The base station and the base station control apparatus of the CDMAsystem as claimed in claim 10, comprising: moving-direction conjecturingmeans for conjecturing a moving direction of said mobile device from thelocational information of said mobile device, wherein said first handoffmeans performs the inter-frequency handoff to the tier of said commonfrequency when the moving direction of said mobile device is outward ofthe frequency-tier boundary.
 12. The base station and the base stationcontrol apparatus of the CDMA system as claimed in claim 11, whereinsaid moving-direction conjecturing means conjectures a detailed movingdirection by combining the locational information of said mobile deviceand cartographic information of said cell conducting the communication.13. The base station and the base station control apparatus of the CDMAsystem as claimed in claim 10, comprising, second handoff means forperforming an inter-frequency handoff from the tier of said commonfrequency to a tier of an uncommon frequency prepared in said cellconducting the communication and not prepared in said adjacent cell,when there is no change in the locational information of said mobiledevice.
 14. The base station and the base station control apparatus ofthe CDMA system as claimed in claim 11, comprising: third handoff meansfor performing an inter-frequency handoff to a tier of an uncommonfrequency prepared in said cell conducting the communication and notprepared in said adjacent cell, when said mobile device is distant fromsaid frequency-tier boundary according to said locational information.15. The base station and the base station control apparatus of the CDMAsystem as claimed in claim 14, comprising: moving-velocity conjecturingmeans for conjecturing a moving velocity of said mobile device from anamount of change of said locational information, wherein said firsthandoff means performs the inter-frequency handoff to the tier of saidcommon frequency when said moving velocity is a high velocity.